The present invention is related to roof control in underground mines such as coal mines, trona mines, and the like.
In the case of longwall mining, merely by way of example, some type of roof support is required at the headgate, tailgate, and other areas. Currently there are extant the following techniques and materials useful for disposition between and support of a mine roof over a floor area: rigid systems, such as the installation of vertical BIG JOHNS or timber posts, which do offer slight yield but which characteristically will break under substantial roof loading; cribbing, whereby pairs of timber lengths are stacked in quadrature, one pair on top of another, between floor and ceiling; DOUGHNUT CRIBBING, constituted by a series of vertically stacked, precision cast, reenforced concrete "doughnut" elements; disc cribbing, comprising vertically stacked concrete discs, and so forth. Cribbing, i.e. or support installations, are used in underground mines at track turnout areas, track entry intersections, tailgate entries, at headgates proximate conveyor installations, and so forth.
It is common knowledge that conventional cribbing is quite expensive and makes transportation, ventilation, and production quite difficult under certain operating conditions. Timber posts such as BIG JOHNS reach peak strength very rapidly but have little or no yield, causing roof and/or floor failures and then, themselves fail under excessive loading.
Longwall mining is very much an automated and integrated continuous mining method; problems which occur as to any part of the mining system can cause stoppage of the whole operation. Should ground control problems arise anywhere, and even though the same be localized, it can have a considerable impact on the whole production process.
The Use of Mine Timbers as Support Posts: While the tensile strength of wood parallel to its grain is extremely high, the crushing strength of a wood support post is, on average, only about fifty percent of its tensile strength along the grain. Such a reduction of strength may be explained by the nature of wood. Additionally, crushed strength is drastically reduced in the presence of appreciable wood moisture.
Finally, and depending upon the character of the wood selected, a wood post will yield only a small percentage figure before the crush point is reached. Failure as to buckling strength likewise comes into play for timber, especially where the ratio of timber length to its diameter is less than eleven. Finally, duration of stresses, i.e. fatigue, is progressive in its effect upon wood timber. Where wood posts are replaced by steel posts which are unitary, then the desired ability to yield progressively is drastically reduced.
Again, as to longwall mining, controlled roof caving is extremely important and some yield as to support posts is needed for roof control during the caving process.
Roof control is likewise necessary to deter failures of underground coal pillars and entries as may be caused by combined effects of induced stresses in surrounding rock and coal and the inability of rocks and coal, of themselves accommodate appropriately.
A skilled engineer, being aware of the nature of the mine and its surrounding formations, will be able to calculate the degree of yield in posts in particular locations that will be required to maintain roof control under a variety of conditions. Such design and accommodation of roof supports, as is made possible in the present invention, can accommodate any one of a host of differing in situ conditions. No art, patent or otherwise, is known which relates to the use of telescoping posts, fabricated from steel as herein set forth and claimed, whereby progressive frictional forces and elastic/plastic deformations allow for and yet delimit post yield in response to roof pressures. The reader, however, should be aware of certain teachings in the following technical publications: (1) COAL VOICE, Jan./Feb. 1990 edition, page 27, article entitled TECH TRANSFER, Artificial Supports increase Safety in Underground Mines, and (2) RI 9279 REPORT OF INVESTIGATIONS/1989, YIELDING STEEL POSTS, by J. P. Donford and L. N. Henton, BUREAU OF MINES, U.S. DEPT. OF INTERIOR. In the first publication two concepts are taught: (1) a yielding steel post with top and bottom legs, the bottom leg having an outer expansion ring and the top leg having a flared lower end which is inserted over the ring, the oversized ring expanding the top leg, the latter yielding radially, during roof-floor convergence. It is noted that, in contrast with the present invention, no provision is made for collapsing the post structure for transit, nor for utilizing wall-friction forces beyond the expansion ring; further, failure such as splitting of the top leg, through excessive radial yield beyond its elastic limit, is likely to occur through plastic deformation to point of rupture. As to the second concept, an expensive, hydraulically actuated yielding post is provided, the same utilizing a pressure lease valve. In the second (2) publication, the mathematics of flow of material through a conical die are developed and importantly, as to concept (1), a chancing of splitting the outer leg in concept (1), supra, is set forth, and the requisite lowering of the steel schedule from schedule 80 to schedule 40, relative to the upper, radially yieldable leg, necessarily reduces the strength of the over-all post structure, constraints not present in the present invention.